Phthalocyanines and metal phthalocyanines show satisfactory photoconductivity and they have been used, for example, in the electrophotographic photoreceptors.
Along with the recent prevalency of laser beam printer or the like using laser beam as a light source instead of conventional incandescent light and providing advantages of increased printing speed, higher image quality and non-impact printing, investigations and developments for the photoreceptors capable of fulfilling the requirements therefor are now in flourish.
Various attempts have been made for using as the optical source semiconductor laser which has remarkably been developed in recent years among other laser beams. In this case, since the wavelength of the semiconductor laser is around 800 nm, those photoreceptors having high sensitivity at the longer wavelength light of around 800 nm are highly demanded.
As the organic photoconductive materials satisfying such requirements, there have been known squaric acid methine dye, cyanine dye, pyrylium dye, thiapyrylium dye, polyazo dye and phthalocyanine dye. Among them, squaric acid methine dye, cyanine dye, pyrylium dye and thiapyrylium dye are deficient in the practical stability such as the repeating use, although it is relatively easy to make them sensitive to the longer wavelength. The polyazo dye has a drawback in that it is difficult to make it possible to absorb the ray of longer wavelength, many steps are required for the production thereof and moreover the separation of impurities is difficult.
While on the other hand, phthalocyanine dye has absorption peaks at the longer wavelength region of higher than 600 nm and the spectral sensitivity may be changed depending on the central metals or crystal forms thereof. Several of them show a high sensitivity at the wavelength region of the semiconductor laser, and research and development on the phthalocyanine dye have extensively been conducted.
The absorption spectrum and the photoconductive property of phthalocyanines are varied not only by the kind of the central metal but also depending on the crystal forms. It has been reported that a specific crystal form of phthalocyanine is selected among those having the same central metal and used as the photosensitive material for use in electrophotography.
The metal-free phthalocyanine was reported that its X-crystal form had high photoconductive property and was sensitive also to the wavelength region of higher than 800 nm, and copper phthalocyanine was reported that its .epsilon.-crystal form was sensitive to the longest wavelength region among many crystal forms thereof. However, the X-form metal-free phthalocyanine which is meta-stable, is difficult to prepare and the crystalline product of stable and constant quality can not be obtained with ease. .epsilon.-form copper phthalocyanine has a spectral sensitivity extended toward the longer wavelength region as compared with .alpha.- and .beta.-copper phthalocyanines, but its sensitivity is abruptly reduced at 800 nm as compared with that at 780 nm and accordingly, it is accompanied with some difficulties in applying the present semiconductor laser which is fluctuating in the lasing wavelength.
In view of the above, many metal phthalocyanines have been examined, and oxyvanadium phthalocyanine, chloroaluminum phthalocyanine, chloroindium phthalocyanine, oxytitanium phthalocyanine, chlorogallium phthalocyanine and magnesium phthalocyanine, etc. have been reported as phthalocyanines having high sensitivity to near infrared light such as the semiconductor laser beam. However, upon using these phthalocyanines as the charge generation material of photoreceptor for use in electrophotography in copying machines or printers, various requirements for the excellent performance have to be met in addition to the sensitivity.
Of the electrical properties, in addition to the high sensitivity to the semiconductor laser beam, superior charge acceptance, small dark decay and low residual potential are required at first and, further, it is required that these properties do not substantially change during repeating use. Particularly, longer duration life of the photoreceptor has recently been considered important and it has been highly demanded that the electrical properties are less varied during the repeating use. In view of the above, no quite satisfactory material has yet been provided.
While the electrical property varies greatly with the metal coordinated to phthalocyanine, the difference in the crystal forms of the identical phthalocyanine also gives a significant change in the electrical properties. For instance, it has been well known for the copper phthalocyanine that there is a great difference in the charge acceptance, dark decay, sensitivity and the like depending on the crystal forms such as .alpha., .beta., .gamma., .epsilon., etc. (Manabu Sawada; Dyestuffs and Chemicals, Vol. 24, No. 6, p 122 (1979)). Further, the spectral sensitivity also changes as the absorption spectrum varies depending on the crystal forms and, among the copper phthalocyanines, .epsilon.-form has the absorption spectrum at the longest wavelength and the spectral sensitivity extended to the longest wavelength region (Isao Kumano; Electrophotography, Vol. 22, No. 2, p 111 (1984)). Such differences in the electrical properties depending on the crystal forms have been known as well in the metal-free phthalocyanine and many other metal phthalocyanines, and much investigatory effort have been made to the preparation of crystal forms with preferred electrical property.
For instance, while a charge carrier generation layer is often prepared from the vapor-deposition film of the metal phthalocyanine, examples of improving the electrical property of the vapor-deposition film by immersing them into an organic solvent such as dichloromethane or tetrahydrofuran or exposing them to the solvent vapor to cause crystal transition have been reported with regard to aluminum, indium and titanium phthalocyanines (refer to each of Japanese Patent Application Laid-Open Nos. 158649/1983, 44054/1984, 49544/1984, 155851/1984 and 166959/1984).
Among them, it is described in Japanese Patent Application Laid-Open Nos. 49544/1984 and 166959/1984 that the oxytitanium phthalocyanine of particular crystal form is applied to the photoreceptor for use in electrophotography. More in detail, Japanese Patent Application Laid-Open No. 49544/1984 describes that oxytitanium phthalocyanine of a crystal form that has intense X-ray diffraction peaks at Bragg angles (2.theta..+-.0.2.degree.) of 9.2.degree., 13.1.degree., 20.7.degree., 26.2.degree. and 27.1.degree. is preferred. An X-ray diffraction spectrum chart is shown, in which several other peaks can be seen and the presence of a peak having the intensity next to the aboves is shown in the range from 7.degree. to 8.degree..
In Japanese Patent Application Laid-Open No. 166959/1984, the charge carrier generation layer is prepared by exposing the vapor-deposition film of the oxytitanium phthalocyanine to a saturated vapor of tetrahydrofuran for 1-24 hours to cause the crystal transition. The X-ray diffraction spectrum thereof indicates that peaks are present by few number and have broad width, and that intense diffraction peaks are shown at Bragg angles (2.theta.) of 7.5.degree., 12.6.degree., 13.0.degree., 25.4.degree., 26.2.degree. and 28.6.degree..
The charge carrier generation layer is formed with these known oxytitanium phthalocyanines mainly by way of vapor-deposition, and the charge generation layer capable of serving to practical use can be prepared only by the procedure of causing crystal transition by exposing the vapor-deposition layer to solvent vapors. However, the vapor-deposition process is not preferred since it requires a higher installation cost and further an increased running cost due to the poor mass productivity as compared with a coating process.
According to the study of the present inventors, it has been found that, in the case of using the oxytitanium phthalocyanine having strong diffraction peaks at Bragg angle of less than 8.degree., the performance of the photoreceptor having the charge carrier generation layer prepared particularly by the coating of the disperse solution thereof is not always satisfactory. This can be seen, for example, also in Example of Japanese Patent Application Laid-Open No. 49544/1984. That is, the photoreceptor formed by the coating has a lower charge acceptance, higher residual potential and a sensitivity reduced by about 40% as compared with those of the photoreceptor formed by the vapor-deposition.
The performance of the photoreceptor using oxytitanium phthalocyanine is thus changed depending on the preparation conditions, because the oxytitanium phthalocyanine has several crystal forms, depending on which the electrical properties are varied.
Accordingly, it is particularly required to examine the method of preparing such crystal form as having satisfactory electrical properties. However, the prior study has mainly directed to the vapor-deposition process of poor mass productivity and not to the preparation conditions for the oxytitanium phthalocyanine of crystal form suitable to the coating process.
In view of the above, the present inventors have made an earnest study and already proposed that the oxytitanium phthalocyanine prepared by suspending dichlorotitanium phthalocyanine in hot water followed by treating with N-methylpyrrolidone was suitable to the coating process (Japanese Patent Application No. 230982/1984).
The oxytitanium phthalocyanine prepared by such a purifying process generally exhibits preferred electrical property more than that obtained by the conventional method. However, as a result of further study of the present inventors, it has been confirmed that at least several types of crystal forms are formed depending on the purification conditions and there is a difference in electrical property also among these crystal forms.
According to the study of the present inventors, it has been found that a crystalline oxytitanium phthalocyanine showing a specific X-ray diffraction pattern which is prepared, particularly, by applying sufficient washing by suspension in hot water till the pH value of the filtrate is adjusted to about 5-7 and further carrying out the treatment with N-methylpyrrolidone sufficiently till the intensity of peaks at Bragg angles of between 4.degree.-8.degree. is reduced to a certain level, is particularly satisfactory in the sensitivity, charge acceptance, dark decay, residual potential and the like and, thus, has well balanced electrical properties. This invention has been accomplished on the basis of the above-mentioned findings.
The object of the present invention is to provide an oxytitanium phthalocyanine of specific crystal form highly sensitive to the near infrared light such as the semiconductor laser beam, excellent in electrical properties and producible without difficulty.
The other object of the invention is to provide a photoreceptor for electrophotography using oxytitanium phthalocyanine mentioned above, which is highly sensitive to the longer wavelength light of around 750 to 800 nm and has preferred electrical properties.
Other objects of this invention will be apparent from the descriptions hereinafter.